Ir conditioning is sensitive, over charging it, under charging it, too much refrigerant oil or contaminants in the system and it will not cool or cool efficiently. Air conditioning works on high pressure only a small leak quickly causes a drop in pressure. It is often hard to locate air conditioning refrigerant leaks. There is a refrigerant that has dye in it that helps locate a leak. After recharging the air conditioning with the dye if the system leaks it leaves a stain to help locate the source of the leak. An electronic "sniffer" can also be used to locate leaks, it acts like a geiger counter as you come closer to the area of the leak the "sniffer" beeps loudly.In either either case the system will need to be evacuated have the leak repaired and recharged with refrigerant and the proper lubricant or the system may soon loose its refrigerant charge again.

I had the same problem. 1st attempt by dealer was to charge the system. 2nd attempt was to replace the compressor and evaporator. Turns out the recirculating flap was stuck allowing heat into blower box. Replaced the valve that controls it, works fine now.
The delaer (Dwight Harrison Stone Mountain GA) was great, total cost for all 3 repairs was < 900.00. The car was 3k out of warranty.